FT-IR microscopes are used to analyze small samples of material. The microscope has a viewing configuration and a measurement configuration. In both configurations the microscope can be used either in a transmitting mode or a reflecting mode, depending upon the nature of the sample. Typically such a microscope is used in conjunction with an IR spectrophotometer. A microscope of this type generally includes a source of visible radiation and can receive analyzing infrared radiation from a source in the spectrophotometer. A typical microscope includes a sample stage for carrying a sample to be investigated and optical elements for guiding radiation from one or other radiation sources to the sample stage. These elements can include a plane mirror, a toroidal coupling optic and a Cassegrain mirror assembly acting as a condenser. A microscope also includes a Cassegrain mirror assembly which images the sample at a given magnification at an intermediate image plane from where the radiation is directed to an infrared detector. The microscope also includes an optical microscope which enables an image sample on the stage to be viewed optically by means of visible radiation and thereby enables areas of interest to be identified. The microscope can also include a video camera which can be used in conjunction with the optical microscope in order to create an image of the sample for display on display means of a computer which is used to control the microscope.
Modern microscopes of this type have a stage which can be moved under computer control to allow several areas of interest to be identified, their coordinates stored and data collected subsequently automatically on the basis of that stored data. Such microscopes also include a variable aperture which can be computer controlled and is located at the intermediate image plane to mask off a portion of the sample. This combined with an oversized single detector element enables the measurement of the infrared spectrum of selected areas of the sample. By stepping the stage and repeating the measurement, the system can slowly build-up a digital image of the sample pixel-by-pixel. An arrangement of this type is described in EP-A-0731371. Typically such microscopes employ a liquid nitrogen cooled, photoconductive mercury cadmium telluride (MCT) element as the infrared detector. A microscope with a single detector has relatively long measurement times and it can take of the order of 10 hours to acquire a 128×128 pixel image.
In order to reduce measurement times, microscopes have been designed which incorporate large detector arrays rather than single detector elements. One known arrangement used in an integrated array of 64×64 liquid nitrogen cooled photovoltaic MCT detectors, each having an area of 60 square microns. This array is capable of acquiring a 64×64 pixel image simultaneously rather than sequentially as in the system referred to above. With such an arrangement it is possible to reduce considerably the measurements times and, for example, a 128×128 map can e acquired in around 5 to 7 minutes. Such arrangements, however, are extremely expensive and typically cost more than 3 times that of a microscope which employs a single detector. Part of this increased cost is due to the cost of the detector itself, which is relatively expensive and another part is attributed to the fact that the slow read out of the multiplexed detector necessitates the use of a sophisticated spectrometer technology called step-scan.
Although such large arrays offer the advantage of speed of measurement through the acquisition of many pixels in parallel, currently available devices suffer from a loss of signal/noise ratio when compared with the projected performance based on a single array element. The loss arises from inefficiencies incurred in the multiplexing needed to handle the signals from such a large number of elements. In addition, the photovoltaic technology used in these arrays results in a reduced wavelength range when compared with the photoconductive devices used as single element detectors.
We have proposed in European Patent Application No 00307372.3 to use a relatively small detector array whose outputs are sufficiently small in number so that they can be processed without the need for complex multiplexing of the outputs.